EP0426357A2 - Récepteur optique à égalisation pour système de communication par ondes optiques - Google Patents

Récepteur optique à égalisation pour système de communication par ondes optiques Download PDF

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Publication number
EP0426357A2
EP0426357A2 EP90311579A EP90311579A EP0426357A2 EP 0426357 A2 EP0426357 A2 EP 0426357A2 EP 90311579 A EP90311579 A EP 90311579A EP 90311579 A EP90311579 A EP 90311579A EP 0426357 A2 EP0426357 A2 EP 0426357A2
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EP
European Patent Office
Prior art keywords
lightwave
signal
fabry
etalon
generating
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Granted
Application number
EP90311579A
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German (de)
English (en)
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EP0426357B1 (fr
EP0426357A3 (en
Inventor
Leonard Joseph Cimini, Jr.
Larry J. Greenstein
Adel A. M. Saleh
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AT&T Corp
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American Telephone and Telegraph Co Inc
AT&T Corp
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Publication of EP0426357A3 publication Critical patent/EP0426357A3/en
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    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/42—Coupling light guides with opto-electronic elements
    • G02B6/4201—Packages, e.g. shape, construction, internal or external details
    • G02B6/4204—Packages, e.g. shape, construction, internal or external details the coupling comprising intermediate optical elements, e.g. lenses, holograms
    • G02B6/4215—Packages, e.g. shape, construction, internal or external details the coupling comprising intermediate optical elements, e.g. lenses, holograms the intermediate optical elements being wavelength selective optical elements, e.g. variable wavelength optical modules or wavelength lockers
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/25—Arrangements specific to fibre transmission
    • H04B10/2507—Arrangements specific to fibre transmission for the reduction or elimination of distortion or dispersion
    • H04B10/2513—Arrangements specific to fibre transmission for the reduction or elimination of distortion or dispersion due to chromatic dispersion
    • H04B10/25133—Arrangements specific to fibre transmission for the reduction or elimination of distortion or dispersion due to chromatic dispersion including a lumped electrical or optical dispersion compensator
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
    • G02B6/293—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means
    • G02B6/29346—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means operating by wave or beam interference
    • G02B6/29358—Multiple beam interferometer external to a light guide, e.g. Fabry-Pérot, etalon, VIPA plate, OTDL plate, continuous interferometer, parallel plate resonator
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
    • G02B6/293—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means
    • G02B6/29379—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means characterised by the function or use of the complete device
    • G02B6/29392—Controlling dispersion
    • G02B6/29394—Compensating wavelength dispersion

Definitions

  • This invention relates to the field of lightwave systems for dispersion equalization in which the system compensates for the effects of dispersion within a transmission medium.
  • An optical equalization receiver for countering the effects of delay distortion, specifically fiber chromatic dispersion is realized by utilizing dynamically controllable Fabry-Perot etalon structures.
  • the associated time delay characteristic of the etalon may be used to counter the effects of delay distortion relating to the fiber chromatic dispersion, thereby allowing higher transmission bit rates or longer transmission distances to be achieved.
  • an optical receiver comprising a reflective Fabry-Perot etalon and a piezoelectric transducer for dynamically controlling the optical path length of the etalon are used in a feedback loop configuration wherein a feedback circuit monitors an output optical signal from an end of the etalon for generating a control signal representing the amount of delay needed to compensate for distortion in an optical signal incident on the etalon.
  • the control signal causes a change in the optical path length of the etalon that results in a shift of the frequency response of the etalon for producing a delay characteristic substantially equal and opposite to the associated delay distortion of the transmission medium.
  • Various feedback control strategies may be used in adjusting the frequency response of the etalon so as to obtain a delay characteristic substantially equal and opposite delay to the associated delay distortion. For instance, in return­to-zero signaling, the etalon frequency response may be adjusted in real time so as to maximize the sinusoidal amplitude component of the detected optical signal at the frequency of the signaling bit-rate. Further, for both retrrn-to-zero and non-return-­ to-zero signaling, another feedback control strategy is to adjust the frequency response of the etalon such that the "eye opening" of the detected optical signal is maximized.
  • the frequency sresponse of the etalon may be adjusted in accordance with the above feedback strategies by changing the optical path length of the etalon, such as by varying the cavity length of the etalon, or by inducing a change in the refractive index of the optical medium enclosed within the cavity of the etalon.
  • a lightwave receiver that compensates for delay distortion in lightwave communication systems is realized by employing a dynamically controllable Fabry­Perot etalon structure.
  • the present invention is based upon the discovery that Fabry-Perot etalons may be used to counteract the effects of fiber dispersion, which effects can be particularly deleterious for optical signals with laser chirp.
  • fiber is observed to have a frequency response constant in amplitude and linear in time delay, with a slope polarity that depends on wavelength.
  • the frequency response of a Fabry-Perot etalon similarly has, over a limited range of frequencies, a substantially linear time delay with either a positive slope or negative slope.
  • a delay characteristic complementary to that of the fiber can be generated for effectively reducing the effects of fiber dispersion over a certain range of frequencies, i.e., wavelengths.
  • optical source 101 is a semiconductor laser operating in a single-longitudinal mode which is directly modulated by a current, 1(t) of the form: where ak is a data sequence of "0" 's and "1" 's, 1 p (t) is the transmitted current pulse and 1/T b is the signaling bit-rate.
  • Any signaling format such as return-to-zero and non-return-to-zero (ZRZ), can be used for transmitting optical signal 100 over a transmission medium.
  • Current waveform, 1(t) is shown to be filtered by pre-source filter 102 to account for laser parasitics as well as for pulse shaping of optical signal 100.
  • Pre-source filter 102 may be, for example, a simple RC circuit or the like.
  • the response of optical source 101 to the filtered current waveform is deterinined by solving the large-signal rate equations, which describe the interrelationship among photon density, carrier density and optical phase within the laser cavity.
  • the laser chirp which is the time derivative of the laser phase, is approximately given by: where a is the linewidth enhancement factor and K is a parameter dependent on the physical structure of the laser.
  • the laser output, E(t) is transmitted over single-mode fiber 104 which is L krti in length.
  • Modeling fiber 104 as a bandpass filter with a flat amplitude response and a linear group delay, the frequency response is given by: where D is the fiber dispersion, ⁇ is the wavelength, c is the speed of light in free space, and w is radian frequency measured from the center frequency of optical source 101.
  • the flat loss of the fiber is irrelevant to this discussion and thus omitted.
  • the slope of the group delay is deterinined by the chromatic dispersion of fiber 104. Not included in the above description are attenuation and higher-order terms in the phase response.
  • optical receiver 112 comprises dynamically controllable etalon structure 105, optical detector 106 and feedback control circuit 107.
  • Feedback control circuit 107 affords a means for adapting the frequency response, H eq ( ⁇ ), of etalon structure 105 by measuring a predeterrnined electrical characteristic of electrical signal 109 and adjusting the etalon frequency response according to a predetermined feedback control strategy so that etalon structure 105 operates at a point on its delay characteristic for producing delay substantially equal and opposite to the associated delay distortion of fiber 104.
  • post-detector filter 108 may be, for example, an n th order Butterworth low-pass filter.
  • the source-to-detector section consists simply of fiber 104 and etalon structurc 105.
  • the impulse response, h f (t), of fiber 104 alone is of the form e -j ⁇ t2 where ⁇ is ⁇ c/ ⁇ 2DL.
  • the phase response is derivable from equation (4) above.
  • the overall impulse response h sys (t) of fiber 104 and etalon structure 105 is desired to have the form of a delta function, i.e., "impulsive". This is because if fiber 104 in combination with etalon structure 105 has an impulse response of the form ⁇ (t), the signal, E(t), propagates unaltered through fiber 104 and etalon structure 105. It is noted that the overall impulse response, h sys (t) is given by h eq (t) * h f (t) or, in the frequency domain, the overall frequency response, H sys ( ⁇ ) is given by H eq ( ⁇ ) ⁇ H f ( ⁇ ).
  • the change in cavity length required to move the response a full period is half of an optical wavelength.
  • the nominal width of the frequency response's primary peak should be approximately less than a bit period. This is achieved through a judicious selection of the mirrors' reflectivity for a given cavity length of the etalon structure.
  • the chirp frequency associated with each of the other peaks of h sys (t) should be large compared to the largest chirp frequency in the laser pulses. This condition ensures that the convolution of secondary impulse response peaks with the input signal is negligible and, moreover, is achieved by choosing the frequency response period of the etalon structure to be substantially greater than the highest chirp frequency.
  • a reflective single-cavity Fabry-Perot etalon and a piezoelectric transducer may be used as dynamically controllable etalon structure 105.
  • back mirror 202 having a reflectively close to 100% and front mirror 201 having a power reflectivity of r2 in combination with optical medium 203 (refractive index ⁇ 1.5) are arranged to form reflective etalon structure 200.
  • a piezoelectric transducer (not shown) may be used as the means for dynamically varying the cavity length of etalon 200.
  • Output optical signal 110 from fiber 104 is coupled optically by lenses (not shown) into fiber 205.
  • Reflective etalon 200 reflects optical signal 110 and, moreover, introduces an associated delay into optical signal 110 to generate optical signal III.
  • Three dB coupler 207 couples optical signal 111 into fiber 206, which directs optical signal 111 to optical detector 106.
  • the reflective etalon and piezoelectric transducer may be implemented as described in U.S. Patent 4,830,451. This reference cited above and its teachings are expressly incorporated herein by reference.
  • an optical circulator instead of 3 dB coupler 205, may be used to couple in and out of reflective etalon 200. See, for example, 1. Tokohama et al., Electron Lett., Vol. 22, No. 7 pp. 370-2(1986).
  • the 3 dB coupler implementation although structurally simpler than the circulator design by Tokohama et al. has a higher loss.
  • Other implementations for reflective etalon structures such as those comprising more than one reflective etalon, may be used instead of a single cavity reflective etalon structure, see R.C. Alferness et al., Electron Lett., Vol. 24, No. 3, pp. 150-1(1988)
  • the frequency response, H eq ( ⁇ ), is given by: where T is the round-trip delay time of the cavity, and A is a constant representing the loss of the structure.
  • T is the round-trip delay time of the cavity
  • A is a constant representing the loss of the structure.
  • the normalized delay response, ⁇ ( ⁇ )/T is found to be periodic in frequency with a period of 1/T Hz and, moreover, from the above equation is given by: It is contemplated that the linear portion of the delay response would be used to counter the delay dispersion of fiber 104. It should be noted that the delay response of etalon 200 should have the appropriate polarity and magnitude to counteract the delay response of the fiber.
  • electrical signal 109 contains a sinusoidal component at the signaling bit-rate 1/T b of optical signal 100.
  • One exemplary method of adaptively controlling the frequency response of etalon 200 is to position the frequency response so as to maximize the amplitude of this sinusoidal component. Such a method approximately corresponds to the narrowest, best-equalized output electrical signal 109. This can be achieved by feedback control circuit 107 periodically measuring the amplitude of the sinusoidal component of electrical signal 109 and using the relative change from previous measurements as a control signal for changing the cavity length of etalon 200 via the piezoelectric transducer.
  • maximizing the amplitude of the frequency component of electrical signal 109 at the signaling frequency, 1/T b may be used as a feedback control strategy, it cannot be used in the case of NRZ signaling. This is because for NRZ signaling, electrical signal 109 contains no frequency component at the signaling bit-rate. However, other feedback control strategies may be used.
  • the "eye opening" versus frequency profile has a convex shape.
  • one method of adaptively controlling the frequency response of etalon 200 is to position the frequency response of the etalon so as to maximize the "eye opening" of electrical signal 109.
  • transmissive etalon structures may be used that utilize a simple, single cavity transmissive etalon, shown in FIG. 4.
  • the frequency response is given by: where the phase response is: and the corresponding delay response is Similar to the previous reflective etalon, r2 is the power reflectivity of mirrors 401, T is the round-trip delay of the cavity and the free spectral range (FSR) is simply given by 1/T.
  • FSR free spectral range
  • optical medium 402 is enclosed witnin mirrors 401. Illustrative amplitude and delay characteristics for this Fabry-Perot etalon are shown in FIGs. 5-6.
  • a simple feedback control strategy may be employed and that is to center the optical signal spectrum of optical signal 110 at the peak of the etalon transmission response. This can be accomplished by maximizing the dc power detected in electrical signal 109. Furthermore, this feedback control strategy works for both NRZ as well as PZ signaling.
  • ⁇ is the wavelength
  • W is the width of the active layer
  • ⁇ ⁇ is the mode confinement factor in the vertical direction
  • Ht is the carrier density required for transparency
  • ⁇ ph is the photon lifetime
  • ⁇ e is the carrier lifetime
  • ⁇ sp is the fraction of spontaneous emission into the lasing mode
  • ⁇ g is the group velocity
  • C is the carrier diffusion coefficient
  • a is the gain coefficient
  • ⁇ is the gain compression coefficient
  • P max is the maximum power output
  • P min is the minimum power output.
  • Pre-source filter 102 in the laser driver circuit had a 3 dB bandwidth of 4 GHz, while post-detector filter 109 had a bandwidth of 6.24 GHz, corresponding to 0.78/Tb where T b is the signaling period.
  • fiber 104 had a dispersion, D, of ⁇ 17 ps/nm/km.
  • Optical signal 100 was generated using a maximal-length pseudorandom sequence of length 64 which contained all bit sequences of length 6.
  • FIGs. 7,8 and 9 Shown in FIGs. 7,8 and 9 are the performance results. It should be noted that these results demonstrate typical improvements that may be obtained by utilizing the present optical equalization receiver.
  • the results show optical signal power penalty versus transmission distance for both NRZ and RZ signaling. In both cases, the optical penalty for an unequalized system is compared to those systems utilizing both reflective and transmissive Fabry-Perot etalon structures.
  • the results in FIGs. 7 and 8 include the effects of laser chirp, while FIG. 9 does not.
  • Reflectivity, r, and cavity delay, T were chosen based on conditions two and three stated hereinabove for obtaining an "impulsive" response. Furthermore, for achieving condition one, the etalon phase response has been positioned by searching over an entire period of the delay characteristic of electrical signal 109 to find that position yielding the maximum "eye opening". The positioning of the phase response results in reducing the fiber delay distortion. In general, sustaining a specified linear phase response over a specified signal bandwidth is not possible regardless of the values of r and T. Nevertheless, the equalization receiver can still provide increased system performance as measured by the optical signal power penalty.
  • the optical penalty is defined as the "eye opening" relative to the case of a fiber having a length zero and no post-detector filter presenL
  • the optical penalty measured at the output of post-detector filter 108 includes chromatic dispersion of the fiber, laser bandwidth limitations and nonlinearities, and receiver bandwidth limitations.
  • the transmissive etalon the parameters correspond to a free spectral range of 80 GHz and a 3 dB bandwidth of 7.6 GHz.
  • equalization significanlly extended the range of "eye-open" operation.
  • FIG. 7 shows that optical equalization can maintain an "open eye” at distances more than twice those distances at which the unequalized system becomes inoperablc.
  • RZ signaling (duty cycle ⁇ 0.75)
  • similar results were obtained, see FIG. 8.
  • the improvements are not as substantial as for signaling, the "eye opening" still remains open for a distance much greater that 100km.
  • Figure 9 shows that in a communication system without laser chirp, utilizing a receiver comprising a controllable reflective etalon structure, the effects of dispersion alone can also be substantially reduced, increasing the achievable transmission distance by at least a factor of two.
  • Figures 10 through 12 show the performance in terms of optical power penalty versus transmission distance for different control feedback strategies used in positioning the frequency response of the etalon structure. It should be recalled that for the reflective etalon structure and the case of RZ signaling, one possible feedback control strategy is to maximize the dc power of electrical signal 109 at the bit-rate frequency, i.e., maximize the first harmonic peak. However, another control strategy is to maximize the "eye opening" of electrical signal 109.
  • Either control strategy is accomplished by adjusting the cavity length of the etalon via the piezoelectric transducer by feedback control circuit 107, which is well known in the prior Feedback control circuit 107 may utilize differentiators, decision circuits, band-pass filters, integrate and dump circuits, envelope detectors and the like for measuring the "eye opening", dc power, or an sinusoidal amplitude component of electrical signal 109.
  • Figure 10 indicates that maximizing the first harmonic peak works reasonably well for distances from 40 km to about 120 km.
  • the etalon response is centered on the center optical frequency of optical signal 110, i.e., at the Fabry-Perot transmission peak.
  • this feedback control strategy works well for both RZ and NRZ signaling.
  • the performance of the lightwave receiver is not very sensitive to variations in the reflectivity or the round trip delay time of the etalon.
  • the optical signal power penalty varies only 0.5 dB for round trip delays between 10 ps and 25 ps. This assumes a fiber length of 100 km, and a reflective etalon structure with reflectivity, r, of 0.74. Also, the optical signal power penalty varies less than 0.2 dB for reflectivities between 0.74 and 0.82 for a fixed round trip delay of 12.5 ps.
  • post-detector filter 108 With consideration to the bandwidth of post-detector filter 108, it was found that the performance of both unequalized and equalized transmission systems was sensitive to the type of post-detector filter and, moreover, its bandwidth. In the results presented, post-detector filter 108 had a bandwidth of 6.24 GHz. For PZ signaling, further computer simulation indicated that increasing the filter bandwidth to 7.55 GHz (r re-optimized to 0.74) reduced the optical power penalty from 3.0 dB to 2.26 dB. Also, different laser parameters could alter the optical power penalty for the given system parameters.
  • the product B2 ⁇ L will be approximately constant for a given optical power penalty.
  • the product B2 ⁇ L increased, for NRZ signaling, from 6400 (Gb/s)2 ⁇ km to better than 25600 (Gb/s)2 ⁇ km and, for RZ signaling, from 5760 (Gb/sec)2 ⁇ km to better than 10240 (Gb/sec)2 ⁇ km.
  • the unequalized system will be inoperable above 25 km, while an equalized system should maintain an "open eye" pattern for fiber lengths greater than 50 km.
  • the midpoint of the "eye opening" may be attained as the threshold setting since it corresponds approximately to the average dc power level at optical detector 106.
  • the frequency component at the signaling frequency of electrical signal 109 may be used to derive the sampling phase for optical detector 106.
  • multiple cavity etalon structures may be used instead of a single cavity structure. See, for example, A. A. M. Saleh et al., Journal of Lightwave Technology, Vol. 7, No. 2 pp. 323-30(1989). Theoretically, these multiple structures should improve the system's performance. Also, the frequency response of the various etalon structures may be adjusted in accordance with the above feedback strategies by inducing a change in the refractive index of the optical medium enclosed within the etalon cavity.

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  • Physics & Mathematics (AREA)
  • Electromagnetism (AREA)
  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • General Physics & Mathematics (AREA)
  • Optics & Photonics (AREA)
  • Optical Communication System (AREA)
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EP90311579A 1989-11-01 1990-10-23 Récepteur optique à égalisation pour système de communication par ondes optiques Expired - Lifetime EP0426357B1 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
US430041 1989-11-01
US07/430,041 US5023947A (en) 1989-11-01 1989-11-01 Optical equalization receiver for lightwave communication systems

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EP0426357A2 true EP0426357A2 (fr) 1991-05-08
EP0426357A3 EP0426357A3 (en) 1992-08-12
EP0426357B1 EP0426357B1 (fr) 1996-04-10

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JP (1) JP2509749B2 (fr)
CA (1) CA2026309C (fr)
DE (1) DE69026465T2 (fr)

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EP0542535A3 (en) * 1991-11-14 1993-09-22 Nec Corporation Method of optical signal communication
EP0542535A2 (fr) * 1991-11-14 1993-05-19 Nec Corporation Méthode de communication par signaux optiques
WO1996031961A1 (fr) * 1995-04-05 1996-10-10 Jds Fitel Inc. Dispositif de compensation de la dispersion chromatique
EP0997751A2 (fr) * 1998-10-30 2000-05-03 Lucent Technologies Inc. Filtre optique passe-tout
EP0997751A3 (fr) * 1998-10-30 2000-12-13 Lucent Technologies Inc. Filtre optique passe-tout
US6289151B1 (en) 1998-10-30 2001-09-11 Lucent Technologies Inc. All-pass optical filters
EP1061674A3 (fr) * 1999-06-14 2004-11-03 Lucent Technologies Inc. Système de communication optique à multiplexage de longueurs d'ondes avec des filtres de compensation de la dispersion accordables multivoies
EP1061674A2 (fr) * 1999-06-14 2000-12-20 Lucent Technologies Inc. Système de communication optique à multiplexage de longueurs d'ondes avec des filtres de compensation de la dispersion accordables multivoies
EP1098212A1 (fr) * 1999-11-05 2001-05-09 JDS Uniphase Inc. Compensateur de dispersion accordable
US6654564B1 (en) 1999-11-05 2003-11-25 Jds Uniphase Inc. Tunable dispersion compensator
US6621614B1 (en) 2001-08-10 2003-09-16 Arista Networks, Inc. Etalons with variable reflectivity
US11303356B1 (en) 2019-04-18 2022-04-12 Raytheon Company Methods and apparatus for maintaining receiver operating point with changing angle-of-arrival of a received signal
US11307395B2 (en) 2019-05-23 2022-04-19 Raytheon Company Methods and apparatus for optical path length equalization in an optical cavity
US11290191B2 (en) 2019-06-20 2022-03-29 Raytheon Company Methods and apparatus for tracking moving objects using symmetric phase change detection
US11159245B2 (en) 2019-07-03 2021-10-26 Raytheon Company Methods and apparatus for cavity angle tuning for operating condition optimization
WO2021011672A1 (fr) * 2019-07-15 2021-01-21 Raytheon Company Procédés et appareil d'accord de longueur de cavité pour optimisation de point de fonctionnement d'un résonateur optique d'un récepteur optique
US11353774B2 (en) 2019-07-15 2022-06-07 Raytheon Company Methods and apparatus for cavity length tuning for operating point optimization

Also Published As

Publication number Publication date
US5023947A (en) 1991-06-11
JPH03169131A (ja) 1991-07-22
CA2026309C (fr) 1995-03-21
DE69026465D1 (de) 1996-05-15
EP0426357B1 (fr) 1996-04-10
EP0426357A3 (en) 1992-08-12
DE69026465T2 (de) 1996-08-08
CA2026309A1 (fr) 1991-05-02
JP2509749B2 (ja) 1996-06-26

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